Blast furnaces are generally fed with hot blast air received from a regenerative heater such as a hot stove or a pebble heater. Such a regenerative heater generally comprises a first column and a second column, fluidly connected at the top by a cupola. A burner is arranged in the lower portion of the first column for burning a fuel and an oxidizing gas. The hot flue gasses created by the burning rise through the first column towards the cupola where they are directed into the second column. The second column is filled with checker bricks for absorbing heat from the hot flue gasses. The flue gasses then exit the second column via an opening in the lower portion of the second column. After the heating cycle, the regenerative heater is switched to a blowing cycle wherein cold air is generally fed into the regenerative heater through the opening in the lower portion of the second column. As the cold air flows through the second column filled with hot checker bricks, heat is transferred from the checker bricks to the cold air, thereby heating up the air. At the top of the second column, the hot air then flows into the first column via the cupola. The hot air finally exits the first column via a blast opening arranged above the burner. The hot air is then fed as hot blast air to the blast furnace.
Such regenerative heaters and their operation are well known to the skilled person and are generally used to heat blast air to a temperature of up to about 1250° C. for injection into the blast furnace. In recent years, the used of top gas recirculation installations has come into the limelight in order to reduce CO2 emissions into the atmosphere. Such top gas recirculation installations recover top gas from the top of the blast furnace and feed the recovered top gas through a recycling process before injecting it back into the blast furnace. The recycling process comprises an initial cleaning of the top gas to remove e.g. dust particles, before the top gas is subjected to a CO2 removal. The top gas is fed through a CO2 removal unit in which CO2 is removed from the top gas, generally by pressure swing adsorption (PSA) or vacuum pressure swing adsorption (VPSA). The CO2 removal unit produces two streams of gas: a CO2 rich tail gas and a CO rich process gas. The CO2 rich tail gas is generally fed through a cryogenic unit to separate pure CO2 out of the CO2 rich tail gas. The pure CO2 is subsequently generally pumped into the ground for storage. The CO rich process gas may be heated and fed back into the blast furnace as reducing gas.
The heating of the CO rich process gas may be carried out in regenerative heaters. The replacement of cold blast air with CO rich process gas, i.e. a reducing gas, however has implications for the operation of the regenerative heaters. Indeed, the oxidizing gas fed to the regenerative heater during the heating cycle is not compatible with the reducing gas fed through the regenerative heaters during the blowing cycle. In order to avoid that oxidizing gas is in the regenerative heater when the reducing gas is fed through, it may be suggested to carry out a purging of the regenerative heater before the blowing cycle is started. Purging the regenerative heater with at least three times its volume however is expensive, time consuming and unnecessarily reduced the temperature of the checker bricks.